Demands for the miniaturization of semiconductor integrated circuits continue to persist, and technical development for reducing the sizes of static random access memories (SRAMs) and logic circuits continues to be carried out. In the present situation, various companies are promoting the development of products with M1 half pitches of 45 nm or less according to their road maps.
In recent years, lax limits on the improvement in exposure devices have become visible. In addition, restraint of variation in device characteristics and improvement in yields associated with miniaturization are mentioned as future tasks.
In order to reduce the physical sizes of circuits to meet the recent demand for miniaturization, there has been increasing importance of not only downsizing lithography patterns but also exercising ingenuity aiming to develop new methods of disposing elements and connecting wiring.
For example, a technique that uses two types of contact plugs of an ordinary contact plug and a shared contact plug has been proposed to connect a transistor forming an SRAM to an upper wiring layer (see Japanese Patent No. 3064999 and Japanese Patent No. 4618914, for example). A shared contact plug can contribute to reducing the area of an SRAM by connecting both a transistor gate electrode and a source/drain region by means of a single contact hole pattern.
On the other hand, improvement in the accuracy of lithographic patterning by variation control is now achieved by forming an SRAM consisting of gate electrodes and active regions both of which have relatively simple line-and-space shapes (see Japanese Patent No. 4618914, for example). It has become increasingly important to exercise ingenuity in pattern shapes for overcoming the limits of lithography.
By way of a method of forming holes, a technique is proposed that uses a two-layer mask pattern and forms holes in portions where openings of these mask patterns overlap each other (see Japanese Unexamined Patent Publication (Kokai) No. SHO 51-051283, Japanese Unexamined Patent Publication (Kokai) No. 2008-211027, Japanese Unexamined Patent Publication (Kokai) No. 2005-159264, and Japanese Unexamined Patent Publication (Kokai) No. 2006-156422, for example).